This invention relates to an adhesive system for adhering an object to a substrate, and in particular relates to an adhesive system that allows for repositioning of the object after its first contact with the substrate to which it is to be adhered.
When an object is adhered to a substrate, commonly it is necessary to reposition the object after it first makes contact with the substrate to which it is to be adhered. One example is in the manufacture of traffic signs, in which the object to be adhered is a piece of plastic sheeting sign material, and the substrate is a sign blank. Another example is the manufacture of plastic barrels that are covered in whole or in part with a brightly colored and/or reflective plastic sheeting material and used to delineate roadway construction work zones. The object to be adhered is plastic sheeting, typically provided as a roll of material, the reverse side of which is provided with a layer of a pressure sensitive adhesive and a release liner, and the substrate is the outer surface of the barrel. To adhere the plastic sheeting material to the barrel substrate, an amount of the sheeting is unwound from the roll, the release liner is peeled off the back, the adhesive side of the sheeting is applied to the substrate, and the piece of sheeting is pressed into place. If the sheeting is not properly aligned on the substrate when the adhesive first contacts the substrate surface, the sheeting must be removed from the substrate and re-applied in proper alignment.
Typically, plastic sheeting materials used on traffic signs and plastic barrels are retroreflective. Such retroreflective sheeting materials can be of the cube corner type as disclosed, for example, in U.S. Pat. No. 4,486,363 (Pricone) and U.S. Pat. No. 4,618,518 (Pricone); or such sheeting materials can be of the glass bead type, as disclosed, for example, in U.S. Pat. No. 3,190,178 (McKenzie) and U.S. Pat. No. 4,025,159 (McGrath). In the case of construction work zone barrels, preferred types of sheeting include those known in the industry as reboundable sheeting products, described in ASTM D 4956. Prior art adhesives for use with retroreflective sheeting include, for example those disclosed in U.S. Pat. No. 4,248,748 (McGrath), U.S. 5,069,964 (Tolliver et al.), U.S. Pat. No. 4,763,985 (Bingham), and U.S. Pat. No. 5,820,988 (Nagaoka). The substrate can be a metal such as aluminum, but in recent years such barrels and sign substrates have been made of plastic, especially polyolefins such as polyethylene and polypropylene. The use of plastic substrates helps to reduce damage or injury that might result from flying debris in the event of a collision with an oncoming vehicle.
The use of plastic substrates has given rise to certain difficulties in adhering the sheeting material to the substrate. Adhesives that have been adequate in the past to adhere retroreflective sheeting to a flat aluminum sign substrate do not form a strong bond between a retroreflective sheeting product and a plastic substrate having a low surface energy, particularly when the substrate is a curved surface, such as the side of a construction zone barrel. A weak bond between the sheeting material and the substrate can create two difficulties. First, the sheeting may tend to suddenly pull away from the substrate upon impact, particularly under freezing weather conditions. Second, expansion and contraction of the plastic substrate during cycles of hot and cold temperatures can create shear forces that can cause the sheeting to delaminate from the substrate.
Attempts to solve this problem in the past have involved the use of extremely tacky and aggressive adhesives, such as certain synthetic rubber formulations with a strong affinity for low surface energy plastics. While the use of such adhesives can provide a satisfactory bond between the sheeting and the substrate, other difficulties can arise in the process of applying the sheeting to the substrate. In particular, if the sheeting is not placed on the substrate in perfect alignment in the first instance, it can not be lifted off the substrate to be repositioned without permanently damaging the sheeting product.
It is one object of the invention to provide an adhesive that will provide a bond between an object and a substrate, and will allow the object to be repositioned after initial contact with the substrate.
It is an object of the invention to provide an adhesive system that will provide a bond between a plastic sheeting material and a plastic substrate, and will allow the plastic sheeting to be repositioned after initial contact with the plastic substrate.
It is yet another object of the invention to provide an adhesive system that will resist sudden separation of the material and the substrate upon impact, particularly under freezing conditions.
It is still another object of the invention to provide an adhesive system that will resist delaminating of the material from the substrate when subjected to shear forces arising from expansion and contraction occurring during thermal cycling.
In accordance with the invention, an adhesive system is provided comprising a first relatively thin layer of adhesive, a second layer comprising a permeable web, and a third layer of adhesive that is optionally thicker than the first layer. The adhesive is preferably one suitable for bonding to low-surface energy materials such as certain plastic substrates. The permeable web is preferably made of a fibrous material, and even more preferably of a fibrous non-woven material.
The adhesive system of the instant invention is applied to a material such that the third layer of the system is disposed against the rear surface of the material to be adhered to the substrate, and such that the relatively thin first layer of the system will be disposed against the surface of the substrate. When the object with the adhesive system on its rear surface is first applied to the substrate, only the relatively thin first layer of the system initially bonds to the substrate surface. Because the tackiness of the first layer is a function of its thickness, the initial bond between the adhesive system and the substrate will be relatively weak so that the object can be lifted off the substrate and reapplied if repositioning is necessary for proper alignment. When the object is on the substrate in the desired position, the object is pressed into place. The applied pressure causes the first relatively thin adhesive layer and the third optionally thicker adhesive layer to come into contact with one another through the second layer comprising the permeable web. The system then acts as a single thick layer of adhesive that provides a very strong bond between the object and the underlying substrate. In an embodiment of the system for use with retroreflective sheeting materials, the resulting bond is strong enough to resist sudden separation of the material from the substrate, even upon impact under freezing conditions. The system also resists delaminating of the material from the substrate, even after the substrate has undergone expansion and contraction due to thermal cycling.